This invention relates to an ultrasonic imaging system with a linear array consisting of several transducer elements which are divided into groups. Each group of commonly controlled transducer elements forms one beam aperture to which one transmitting system and one receiving system is allocated. Each group is equipped with electronic focusing through phase delay for its sound field and with a parallel scanning of the sound field by electronic advancement.
During non-destructive material testing with ultrasound, focusing modulators are more often used for determining the extent of imperfections because of the improved resolution they provide. However, the modulators require different test probes with appropriate focal distances at different depths of imperfection investigation. Thus substantial savings of testing time and costs can be achieved with the introduction of an array test probe because it permits electronic focusing at different depths of the material.
Systems employing the technique of horizontal rows of transmitters can be roughly divided into three groups, i.e. image converters, actual or real-time scanners and phase-modulated arrays of transmitters or phased arrays.
Image converters are systems in which the image of an object penetrated by sound is pictured on a linear or planar array by means of acoustic lens. The stationary distribution of sound pressure across the plane of the array is scanned by cycling through one element after the other. Since the resolution and the field of view depend on the size and number of the elements, the number of individual elements for image converters becomes relatively large, typically several thousands. The acoustic lens/image converter system, in other words, registers the stationary distribution of the sound pressure in an object plane that is parallel to the array side.
The real-time scanner, on the other hand, operates in the pulse/echo mode. The test probe is a linear array, whose elements are activated one after the other, individually or in groups. From the magnitude of the echo and from the delay time, a B-image is created on an intensity-modulated oscilloscope. The width of the image is determined by the length of the array.
Phased-arrays and their test probe, in contrast to the real-time scanner, generally have a smaller number of elements and all of the individual modulators contribute to the formation and control of the sound field. Because of the sweeping movement of the beam, a relatively large, sector-shaped B-image is obtained with this technique in spite of the shortness of the test probe.
An ultrasonic imaging system for examination of pressurized reactor containers is known as a phase-modulated pulse-echo system which, in addition to A-, B- and C-scans provides the possibility of holographic examinations. The entire system is controlled by a PDP 11/34 computer. The pulse-echo operation is used for rapid error search and localization. The acoustic halography then provides more accurate information concerning areas that are suspect of imperfections. The test probe contains a linear array of 120 transmitting and receiving elements which are arranged in two adjacent rows. The system operates at an ultrasonic frequency of 2.3 MHz and the element spacing is .lambda./2 for density waves and .lambda. for shear waves. Every element has its own pulse generator. A transmitting and a receiving system is allocated to these transmitting and receiving elements. For pulse-echo operation, 16 of the 120 elements are always combined during the transmitting phase; during the receiving phase, only 8 elements of this group are operating. During the transmitting phase, the beam aperture consists of 16 elements which are connected to the 120 elements by means of a switching matrix. The receive signals are routed via a switching matrix, pre-amplified, converted from analog to digital signals, appropriately delayed by means of a shift register and reconverted to analog signals. The testing speed of welding seams is up to 1/2 square meter/min (W. Gebhard:--"Grundlagen, Technik und Anwendung in der Werkstoffpruefung", IzfP--Bericht Nr. 770117-TW, pages 60 through 65, Saarbruecken 1977).
In this well-known ultrasonic imaging system, the selection of the array is implemented by means of a switching matrix. However, it does not seem possible to create a switching matrix consisting of several thousands switching matrix points, and which still functions well at high ultrasonic frequencies.